embryology from fertilization to gastrulation. 2 fertilization
TRANSCRIPT
EmbryologyFrom Fertilization to Gastrulation
2
Fertilization
3
Fertilization
Steps of fertilization (Conception). Several sperm penetrate corona radiata. Several sperm attempt to penetrate
zona pellucida. One sperm enters egg and nuclei fuse,
producing a zygote. Egg’s plasma membrane and zona
pellucida change to prevent polyspermy.
Fertilization
Compaction starts
First Days of Development Day 0-5
First Days of Development Day 6
7
Occurrence of Pregnancy
When a zygote begins dividing, it is termed an embryo (pre-embryo). Developing embryo travels down
oviduct and eventually implants in endometrium. (Implantation / pregnancy) Presence of human gonadotropic
hormone (HCG) in the blood confirms pregnancy. If implantation does not occur, a
woman never knows fertilization took place.
8
Human Development before Implantation
9
Pre-Embryonic and Embryonic Development Processes of development.
Cleavage - Cell division without growth. Growth - Increase in size of cells. Morphogenesis - Shaping of embryo. Differentiation - Cells take on specific
structure and function.
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Extraembryonic Membranes
Membranes that extend out beyond the embryo. Amnion - Provides fluid environment for
developing embryo and fetus. Yolk sac - First site of red blood cell
formation. Allantois - Contributes to cardiovascular
system. Chorion – outermost membrane,
develops from the trophoblast, contributes to the placenta.
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Extraembryonic Membranes
12
Pre-Embryonic and Embryonic Development Stages of development.
Morula - Solid mass of cells resulting from cleavage.
Blastocyst (Blastula) - Ball of cells formed from morula. Embryonic disk - Inner mass of cells of
blastocyst. Gastrula - Embryo composed of three tissues.
Ectoderm, mesoderm, endoderm. These are the foundation of the body
systems (Germ Layer Theory).
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Early Developmental Stages
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Pre-Embryonic and Embryonic Development Stages of development.
Neurula - Nervous system develops from ectoderm located just above the notochord. Involves induction as one tissue influences
the development of another tissue.
Review Question.
Which of the following is the origin of the mitochondrial DNA of all human Adult Cells?
A) Paternal onlyB) Maternal onlyC) A combination of paternal and maternalD) Either paternal or maternalE) Unknown origin
Review Question.
Which of the following is the origin of the mitochondrial DNA of all human Adult Cells?
A) Paternal onlyB) Maternal onlyC) A combination of paternal and maternalD) Either paternal or maternalE) Unknown origin
First Days of Development Day 7
Week TwoDays 8
Day 9
Day 10
Day 11
Day 12
Days 12-13
Chorion membraneconsists of?
Days 13
Day 14-15
Primary ectoderm:primordial germ cells, endothelial cells, andhematopoietic stem cells.
Extraembryonic mesoderm of the yolk sac wall is a major site of hematopoiesis.
Chorion membraneconsists of?
Origin of the germ lineA. Migration of the primordial germ cells.
B. Migration into posterior body wall.
Clinical ApplicationHuman chorionic gonadotropin (hCG) hCG is a 57,000 MW glycoprotein with two subunits
(alpha and beta) produced by the syncytiotrophoblast.
Enters the maternal blood circulation.
Prevents degeneration of the corpus luteum.
Stimulates production of progesterone in the corpus luteumand chorion, which sustains the placenta.
Can be assayed in maternal blood at day 8 after fertilizationAnd in maternal urine at day 10. This is the basis of early diagnosis of pregnancy.
Clinical ApplicationComplete hydatidiform mole
Persistent trophoblastic diseaseMetastatic choriocarcinoma
IV. Clinical Applications.F. Genomic Imprinting.1. Cytogenetic analysis of hydatidiform moles suggests
Paternal genetic complement->placental development.Maternal genetic complement->embryo development.
2. Methylation of DNA is a mechanism that leads to independentexpression of maternal and paternal genomes duringearly development. Female germ line highly methylated.
3. Example of pattern of inheritance:Father->Prader-Willi SyndromeMother->Angelman Syndrome
4. Severity and age of onset of several genetic diseases also differon inheritance pattern.
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Week 2 – The Two-Layered Embryo Bilaminar embryonic disc – inner
cell mass divided into two sheets Epiblast and the hypoblast
Together they make up the bilaminar embryonic disc
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Week 2 – The Two-Layered Embryo Amniotic sac – formed by an
extension of epiblast Outer membrane forms the amnion Inner membrane forms the amniotic
sac cavity Filled with amniotic fluid
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Week 2 – The Two-Layered Embryo Yolk sac – formed by an extension of
hypoblast Digestive tube forms from yolk sac NOT a major source of nutrients for
embryo Tissues around yolk sac
Gives rise to earliest blood cells and blood vessels
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Figure 3.4 (1 of 3)
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Figure 3.4 (2 of 3)
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Figure 3.4 (3 of 3)
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Week 3 – The Three-Layered Embryo Primitive streak – raised groove on
the dorsal surface of the epiblast Gastrulation – a process of
invagination of epiblast cells Begins at the primitive streak Forms the three primary germ layers
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Week 3 – The Three-Layered Embryo Three Germ Layers*
Endoderm – formed from migrating cells that replace the hypoblast
Mesoderm – formed between epiblast and endoderm
Ectoderm – formed from epiblast cells that stay on dorsal surface
*All layers derive from epiblast cells!
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The Primitive Streak
Figure 3.5e–h
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The Notochord
Primitive node – a swelling at one end of primitive streak Notochord forms from primitive node
and endoderm Notochord – defines body axis
Is the site of the future vertebral column Appears on day 16
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Formation of the Mesoderm and Notochord
Figure 3.6
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Neurulation
Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal
midline thickens Neural groove – ectoderm folds inward
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Neurulation
Neurulation (continued) Neural tube – a hollow tube pinches off
into the body Cranial part of the neural tube becomes
the brain Maternal folic acid deficiency causes
neural tube defects
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Neurulation
Neural crest Cells originate from ectodermal cells Forms sensory nerve cells
Induction Ability of one group of cells to influence
developmental direction of other cells
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The Mesoderm Begins to Differentiate Somites – our first body segments
Paraxial mesoderm Intermediate mesoderm – begins
as a continuous strip of tissue just lateral to the paraxial mesoderm
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The Mesoderm Begins to Differentiate Lateral plate – most lateral part of
the mesoderm Coelom – becomes serous body cavities
Somatic mesoderm – apposed to the ectoderm
Splanchnic mesoderm – apposed to the endoderm
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Third week of development Begins 6 week period of rapid development
and differentiation Gastrulation
1st major event of 3rd week – about 15 days Bilaminar embryonic disc transforms into
trilaminar embryonic disc Ectoderm (skin and nervous system), mesoderm
(muscle, bones, connective tissues, peritoneum), and endoderm (epithelial lining of GI tract, respiratory tract, and several other organs)
Involves rearrangement and migration of epiblast cells Primitive streak establishes head (primitive node) and
tail ends
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Gastrulation
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Third week of development (cont.)
Gastrulation (cont.) 16 days after fertilization notochord forms –
induces tissue to become vertebral bodies 2 depressions form
Oropharyngeal membrane will later break down to connect mouth to pharynx and GI tract
Cloacal membrane will later degenerate to form openings of anus, urinary and reproductive tracts
When cloacal membrane appears, wall of yolk sac forms allantois Extends into connecting stalk In most other mammals used for gas exchange and
waste removal – human placenta does this instead Does function in early formation of blood and blood
vessels and urinary bladder
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Development of the notochordal process
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Third week of development (cont.) Neurulation
Notochord also induces formation of neural plate Edges of plate elevate to form neural fold Neural folds fuse to form neural tube Develop into brain and spinal cord Neural crest cells give rise to spinal and cranial nerves
and ganglia, autonomic nervous system ganglia, CNS meninges, adrenal medullae and several skeletal and muscular components of head
Head end of neural tube develops into 3 primary brain vesicles Prosencephalon (forebrain), mesencephalon
(midbrain), and rhombencephalon (hindbrain)
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Third week of development (cont.) Development of somites
Mesoderm adjacent to notochord and neural tube forms paired longitudinal columns of paraxial mesoderm
Segment into paired, cube-shaped somites Number of somites can be correlated to age
of embryo Each somite has 3 regions
Myotome – develops into skeletal muscles of neck, trunk and limbs
Dermatome – develops into connective tissue Sclerotome - develops into vertebrae and ribs
Development of intraembryonic coelom Splits lateral plate mesoderm into
Splanchnic mesoderm – forms heart, blood vessels, smooth muscle and connective tissues of respiratory and digestive systems
Somatic mesoderm – gives rise to bones, ligaments, dermis of skin
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Neurulation and the development of somites
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Third week of development (cont.) Development of cardiovascular system
Angiogenesis – formation of blood vessels Spaces develop in blood islands to form lumens of blood
vessels Pluripotent stem cells form blood cells By end of 3rd week, heart forms and begins to beat
Development of chorionic villi and placenta Chorionic villi – fingerlike projections of chorion
projecting into endometrium Blood vessels in chorionic villi connect to embryonic
heart through body stalk (becomes umbilical cord) Maternal and fetal blood do not mix – diffusion only
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Development of chorionic villi
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Placentation Process of forming placenta
By beginning of 12th week has 2 parts Fetal portion formed by chorionic villi of chorion Maternal portion formed by decidua basalis of
endometrium Functionally allows oxygen and nutrients to diffuse
from maternal to fetal blood while carbon dioxide and wastes diffuse from fetal to maternal blood
Not a protective barrier – allows microorganisms, drugs, alcohol to pass
Connection between embryo and placenta through umbilical cord 2 umbilical arteries carry deoxygenated fetal blood to
placenta 1 umbilical vein carries oxygenated blood away from
placenta Afterbirth – placenta detaches from uterus
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Placenta and umbilical cord
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Fourth week of development
4th -8th week - all major organs develop Organogenesis – formation of body organs and
systems Embryo triples in size this week Converted from flat disc to 3D cylinder through
embryonic folding Main force is different rates of growth for different
parts Head fold brings heart and mouth into eventual
adult position Tail fold brings anus into eventual adult
position Lateral folds for primitive gut – forerunner of GI
tract
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Embryonic folding
I. somite/Nt development.
I
Axial skel. Vol muscl. Dermis
Urinary systemGenital system
SplanchnopleuricSomatopleuricmesoderm
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Week 3 – The Three-Layered Embryo Primitive streak – raised groove on
the dorsal surface of the epiblast Gastrulation – a process of
invagination of epiblast cells Begins at the primitive streak Forms the three primary germ layers
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Week 3 – The Three-Layered Embryo Three Germ Layers*
Endoderm – formed from migrating cells that replace the hypoblast
Mesoderm – formed between epiblast and endoderm
Ectoderm – formed from epiblast cells that stay on dorsal surface
*All layers derive from epiblast cells!
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
The Primitive Streak
Figure 3.5e–h
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
The Notochord
Primitive node – a swelling at one end of primitive streak Notochord forms from primitive node
and endoderm Notochord – defines body axis
Is the site of the future vertebral column Appears on day 16
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 3.6
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Neurulation
Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal
midline thickens Neural groove – ectoderm folds inward
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Neurulation
Neurulation (continued) Neural tube – a hollow tube pinches off
into the body Cranial part of the neural tube becomes
the brain Maternal folic acid deficiency causes
neural tube defects
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Neurulation
Neural crest Cells originate from ectodermal cells Forms sensory nerve cells
Induction Ability of one group of cells to influence
developmental direction of other cells
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
The Mesoderm Begins to Differentiate Somites – our first body segments
Paraxial mesoderm Intermediate mesoderm – begins
as a continuous strip of tissue just lateral to the paraxial mesoderm
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
The Mesoderm Begins to Differentiate Lateral plate – most lateral part of
the mesoderm Coelom – becomes serous body cavities
Somatic mesoderm – apposed to the ectoderm
Splanchnic mesoderm – apposed to the endoderm
76
Germ Layer Theory of Organ –System Development